1. Field of the Invention
The present invention relates generally to implantable medical devices, and more particularly to implantable leads. Even more particularly, the invention relates to an abrasion and tear resistant implantable lead.
2. Related Art
Implantable leads deliver electrical therapy to a patient's heart. Such a lead is coupled at one end to an implantable medical device, such as, for example, an implantable cardioverter defibrillator (ICD) or a pacemaker. These devices are generally known as pulse generators.
The implantable lead when coupled to an ICD, delivers therapy in the form of an electrical current to the heart in an attempt to correct a detected cardiac arrhythmia. Heartbeat irregularities are fairly common and many are harmless. A severe heartbeat irregularity known as a ventricular tachycardia is an abnormally rapid heartbeat. An implantable lead and an ICD are designed to work together to apply such therapy automatically and quickly to minimize damage to the heart.
ICDs and pacemakers monitor and deliver pacing pulses to a patient's heartbeat through a pacing/sensing electrode of the lead and an ICD delivers high voltage electrical pulses through a defibrillation electrode of the lead. The housing of implantable medical device or pulse generator, such as an ICD or a pacemaker, is commonly referred to as a case or "can."
Implantable leads, whether epicardial or endocardial, are thus used to deliver electrical pulses from a pulse generator to a patient's heart. Such leads are coupled to one or more terminals on the pulse generator at one end, known as the proximal end, by connector pins on one or more legs of the lead. On the distal end of the lead, one or more electrodes are used to deliver electrical therapy to the patient's heart. In the case of a bifurcated or trifurcated lead, a yoke connects the legs to the lead body. The implantable lead includes one or more electrical conductors or wires extending along the length of the lead and electrically connecting the proximal lead connectors to the electrodes. The conductors of the lead are surrounded by a flexible, electrically resistant material such as silicone or polyurethane, referred to as insulation.
The lead is typically implanted through a vein (the cephalic and subclavian veins are the most common) near the patient's neck and threaded down to the heart. The proximal end of the lead is then tunneled under the facia to a pocket in the pectoral region created by an incision in the patient's chest. After testing of the leads to ensure proper placement, the pulse generator is then surgically implanted into the patient's chest, in the pocket.
Unfortunately, conventional implantable leads are susceptible to insulation defects and fractures from frictional contact between the lead and pulse generator or from contact between the leads. The exterior of the pulse generator is usually made from a metal, such as, for example, titanium. Movement of the titanium pulse generator against an implantable lead can abrade the lead's insulation, ultimately exposing conductors resulting in lead failure. Failure of a rate-sensing lead could cause, for example, a defibrillator to misidentify a patient's fibrillating heart (i.e., failure to sense), deliver inappropriate therapy (e.g., misidentify a normal sinus rhythm as fibrillation), or even cause ventricular fibrillation (VF).
Such lead failure is well documented. See DeLurgio D. B., Sorrentino D. M., Leon A. R., Langberg J. J., Implanted Cardioverter Defibrillator (ICD) Lead Abrasion is a Universal Problem, Emory University Hospital, Atlanta, Ga. Supplement I, Circulation Vol. 94, No. 8, p. 564, Oct. 15, 1996, and DeLurgio, D. B., Sathavorn C., Mera F., Leon A., Walter P. F., Langberg J. J., Incidence and Implications of Abrasion of Implantable Cardioverter-Defibrillator Leads, Emory University Hospital, Atlanta, Ga., Jan. 28, 1997, the contents of which are incorporated herein by reference in their entirety.
When a lead is transvenously implanted, the lead is sutured into place and the excess lead length can be wrapped in a loop and be placed adjacent to, e.g., behind and against, the pulse generator. Friction between the lead and the pulse generator's case can result in lead insulation defects, i.e. breaks in the insulation exposing one or more conductors. A short circuit can occur between the conductor at the site of the insulation defect and the electrically active case of the pulse generator resulting in a high current flow directed to a small area of the pulse generator. Thus, potential damage to the internal medical device circuitry can warrant generator replacement upon identification of lead defects. See Gummert J., Krauss B., Hutschenreiter W., Hambrecht R., Mohr F. W., Sensing Lead Insulation Defect Resulting in a Damage of the ICD Pulse Generator Case, Department of Cardiac Surgery, Department of Cardiology University Leipzig, Leipzig, Germany, Pace, Vol. 21, pp. 478 and 479, February, 1998, the contents of which is incorporated herein by reference in its entirety.
Inevitable, eventual, battery depletion requires removal and replacement of the pulse generator by surgery. For cosmetic reasons, to avoid multiple incision scars, it is preferable for pulse generator replacement that the prior incision be carefully reincised by the surgeon. Reincision however, requires that the surgeon take great care to protect the underlying leads. Unfortunately, reincision could result in inadvertent damage via scalpel nicks and cuts to the silicone insulation of the leads, requiring repair or replacement of the leads. When cutting away adhered tissue, scalpel nicks of silicone can propagate into full thickness tears. Lead inspection and replacement may at times be necessary. During routine device replacements due to battery depletion, as already discussed, or device upgrades, e.g., single to dual chamber and to smaller size pulse generators, the end of the lead which attaches to the pulse generator is exposed for examination of any abrasions caused by the pulse generator. If extensive lead abrasions are found, the lead must be extracted and replaced. Removal and replacement of a lead is both costly and potentially risky. Alternatively, the insulation of the lead could be patched by a messy and unreliable technique of manually patching the lead with room temperature vulcanization (RTV) biocompatible silicone material. Abandonment of leads is also widely practiced, but has drawbacks as well.
Currently no device-related preventative measures (such as reinforced silicone) to address lead abrasion are commercially available. Silicone has a superior thirty year reputation of reliability, but has relatively poor abrasion and tear resistance. Polyurethane is more resistant to cuts or tears, but historically has had biodegradation problems. Biodegradation properties of materials include environmental stress crack resistance (ESCR) and the propensity to exhibit metal ion oxidation (MIO). ESCR refers to the propensity of a polymer to resist degrading when stressed and also particularly when in a highly acid or oxidative environment. ESCR is a mechanism by which plastics fail by small cracking or crazing. MIO refers to a property of lead insulation material, by which ions have been implicated with inducing cracking in the insulation of the lead material, due to the conductors used in the lead.
What is needed then is an improved, tear and abrasion resistant silicone insulated lead.